DESCRIPTION

setreuid() sets real and effective user IDs of the calling process.
Supplying a value of -1 for either the real or effective user ID forces
the system to leave that ID unchanged.
Unprivileged processes may only set the effective user ID to the real
user ID, the effective user ID, or the saved set-user-ID.
Unprivileged users may only set the real user ID to the real user ID or
the effective user ID.
If the real user ID is set or the effective user ID is set to a value
not equal to the previous real user ID, the saved set-user-ID will be
set to the new effective user ID.
Completely analogously, setregid() sets real and effective group ID's
of the calling process, and all of the above holds with "group" instead
of "user".

RETURNVALUE

On success, zero is returned. On error, -1 is returned, and errno is
set appropriately.

ERRORS

EPERM The calling process is not privileged (Linux: does not have the
CAP_SETUID capability in the case of setreuid(), or the
CAP_SETGID capability in the case of setregid()) and a change
other than (i) swapping the effective user (group) ID with the
real user (group) ID, or (ii) setting one to the value of the
other or (iii) setting the effective user (group) ID to the
value of the saved set-user-ID (saved set-group-ID) was
specified.

CONFORMINGTO

POSIX.1-2001, 4.3BSD (the setreuid() and setregid() function calls
first appeared in 4.2BSD).

NOTES

Setting the effective user (group) ID to the saved set-user-ID (saved
set-group-ID) is possible since Linux 1.1.37 (1.1.38).
POSIX.1 does not specify all of possible ID changes that are permitted
on Linux for an unprivileged process. For setreuid(), the effective
user ID can be made the same as the real user ID or the save set-user-
ID, and it is unspecified whether unprivileged processes may set the
real user ID to the real user ID, the effective user ID, or the saved
set-user-ID. For setregid(), the real group ID can be changed to the
value of the saved set-group-ID, and the effective group ID can be
changed to the value of the real group ID or the saved set-group-ID.
The precise details of what ID changes are permitted vary across
implementations.
POSIX.1 makes no specification about the effect of these calls on the
saved set-user-ID and saved set-group-ID.
The original Linux setreuid() and setregid() system calls supported
only 16-bit user and group IDs. Subsequently, Linux 2.4 added
setreuid32() and setregid32(), supporting 32-bit IDs. The glibc
setreuid() and setregid() wrapper functions transparently deal with the
variations across kernel versions.